Effects And Physiological Mechanisms Of Priming On Cross Stress Tolerance In Wheat

Abiotic stress(such as drought,waterlogging,high temperature,low temperature,etc.)is an important factor limiting the growth and yield of wheat.Plants can respond to future stress more actively by applying moderate stress in advance,this process of improving plant stress tolerance is called “priming”.In this study,Yangmai 16 was used to study the effects of priming on wheat cross stress tolerance and its physiological mechanisms under hydroponic and pot conditions.The experiments were set as follows:(1)The hydroponic experiments: wheat seedling grown hydroponically in Hogland solution were subjected to drought priming(D,10% PEG6000,-0.6 Mpa,24 h),low temperature priming(L,day and night temperature was set to 10℃/8℃,24 h),high temperature priming(H,day and night temperature set to 30℃/24℃,48 h)and waterlogging priming(W,continuous nitrogen gas in solution,dissolved oxygen 0.22 mg/L,48 h)were conducted at the three leaf stage.Then returned to normal growth conditions immediately after treatment(day and night temperature was set to 18℃/16℃,dissolved oxygen 12 mg/L.Then the drought stress(DD,LD,HD,WD,20% PEG6000,-0.97 Mpa,24 h),low temperature stress(DL,LL,HL,WL,climate chamber day and night temperature was set to 2℃/0℃,24 h),high temperature stress(DH,LH,HH,WH,climate chamber day and night temperature was set to 35℃/30℃,48 h),waterlogging stress(DW,LW,HW,WW,continuous nitrogen gas in solution,dissolved oxygen0.15 mg/L,120 h)were conducted at the five leaf stage,to analysis the effects of priming on cross stress tolerance of wheat and mechanisms of photosynthetic and antioxidant capacity,osmotic regulation and hormone response.(2)Potted experiements: drought priming(soil relative water content 55%-65%,48 h)was carried out during the five-leaf stage of wheat,then severe drought stress(soil relative water content 45%-55%,5 d),waterlogging stress(keeping 1-2 cm water layer,5 d)were carried out during the filling stage(7 days after flowering).The potted experiements were conducted to study the effects of drought priming on water stress tolerance of wheat at grain filling stage and its physiological mechanisms.The main results are as follows:1.Priming enhance stress tolerance of wheatCompared with non-primed treatment,priming improved dry matter accumulation,net photosynthetic rate and actual photochemical efficiency of wheat,indicating that priming improved photosynthetic capacity of wheat under cross stress,increased dry matter accumulation and improved stress tolerance.According to the total dry weight,the priming effect was quantified as follows: DD > WW > HD > DW > LD > WL > DH > WH > LW > DL >HW > LL > HH > WD > HL > LH,but the effects of WD,HL,LH on dry matter effect were not significant.The order of mitigation effect of different stress priming on waterlogging stress was water-logging priming > drought priming > low temperature priming > high temperature priming;the order of mitigation effect on high temperature stress was drought priming > waterlog-ging priming > high temperature priming;the order of mitigation effect on drought stress was drought priming > high temperature priming > low temperature priming;the order of mitigation effect on low temperature stress was waterlogging priming > drought priming > low temperature priming.2.Photosynthetic and antioxidant mechanism of priming enhancing wheat cross stress toleranceCompared with non-primed plants,the order of malondialdehyde(MDA)reduction in priming wheat leaves was DL(42.2%)> LL(22.9%)> LW(21.9%)> DD(21.8%)> DW(20.4%)> DH(20.1%)> HW(19.8%)> WH(18.8%)> LH(15.9%)> WW(15.5%)> WL(15.1%),while HD,LD,WD,HL,HH had no significant difference with non-primed plants.The order of MDA reduction in wheat roots was LL(34.2%)>WW(23.2%)> LD(20.9%)>LW(20.6%)> DD(17.5%)> LH(17.4%)> HW(10.0%)> HL(9.4%),while WL,DL,HD,WD,DW,HH,WH and DH did not significantly reduce MDA content.On the whole,in terms of membrane lipid peroxidation damage,drought priming had a mitigation effect on wheat leaves under low temperature,drought,waterlogging and high temperature stresses,low temperature priming had a mitigation effect on wheat roots under cross stresses,and waterlogging,low temperature and high temperature priming had a mitigation effect on wheat leaves and roots under waterlogging stress,these mitigation effects were mainly due to the improvement of activities of antioxidant-related enzymes superoxide dismutase(SOD),glutathione reductase(GR)and reduced glutathione/oxidized glutathione(GSH/GSSG)under stress.3.Osmotic regulation and hormone response mechanism of priming enhancing wheat cross stress toleranceIn the study of osmotic adjustment response of wheat stress tolerance enhanced by priming,the osmotic adjustment response of a kind of priming to cross stress was as follows:Compared with non-primed plants,drought priming mainly improved stress tolerance by increasing the contents of total soluble sugar(42.9%,30.5%,23.3% and 10.1%)and sucrose(27.3%,12.6%,9.3% and 26.3%)of leaves in DW,DL,DD and DH treatment;low temperature priming mainly increased the total soluble sugars(43.3%,82.1% and 5.8%)of leaves in LW,LD and LL treatment to enhance waterlogging,drought and cold tolerance.In addition,under low temperature stress,the drought,waterlogging and low temperature priming sig-nificantly increased the contents of soluble total sugar and proline in wheat leaves and root sucrose content;under high temperature stress,the drought,waterlogging and high tempera-ture priming increased the contents of sucrose and fructose in wheat leaves;under waterlogging stress,the stress priminng increased the total soluble sugar content of wheat;under drought stress,the drought,waterlogging,low tem-perature and high temperature priming increased the contents of soluble total sugar and root proline(90.6%,45.6%,48.2%,89.6%).It indicated that priming mainly adapted to temperature stress by regulating the content of osmotic adjustment substances in plant leaves,and adapted to water stress by regulating the osmotic adjustment substances in plant leaves and roots.In the study of hormone response of priming to enhance stress tolerance of wheat,compared with non-primed plants,the order of ABA increment in wheat leaves after priming was:HW(100.0%)> DW(39.4%)> HL(35.4%)> WW(23.8%);the order of IAA increment in leaves was: HW(99.6%)> DW(71.0%)> WH(59.4%)> HL(56.2%)> DD(51.5%)> WW(49.1%)> HD(28.0%)> WL(22.1%)> LH(21.2%)> DH(19.9%).The order of ABA increment in wheat roots was WW(37.6%)> LD(37.56%)> HH(36.6%)> LH(32.1%)>DW(31.9%)> DL(22.8%);the order of IAA increment was WD(91.3%)> LD(57.8%)>HD(45.9%)> WH(22.4%).It indicated that drought,waterlogging and high temperature priming participated in the formation of stress tolerance in wheat under cross stress by regulating different hormones in different parts,while priming mainly increased ABA and IAA contents in wheat leaves under waterlogging stress and IAA content under drought stress.4.Drought priming enhanced wheat tolerance to drought and waterlogging stress during grain fillingDrought priming can improve the stress tolerance of wheat under drought and waterlogging stress at grain filling stage.Compared with non-primed plants,drought priming can improve the contents of soluble total sugar(16.9%,7.5%),sucrose(32.0%,16.2%)and fructose(14.5%,18.1%),proline content(82.1%)in wheat leaves,meanwhile,it increased cell water absorption and protected cells from damage.In addition,drought priming increased the activities of SOD,CAT,APX and GPX,reduced the content of reactive oxygen species in cells and membrane lipid peroxidation damage,increased the accumulation of dry matter and finally improved wheat yield.In summary,compared with non-primed plants,drought priming improved the tolerance of wheat under cross stress,waterlogging priming improved the heat tolerance,cold tolerance and waterlogging tolerance of wheat,high temperature priming improved the heat tolerance,waterlogging tolerance and drought tolerance of wheat,and low temperature priming improved the cold tolerance,waterlogging tolerance and drought tolerance of wheat.These priming can maintain the redox balance of cells by improving the photosynthetic and antioxidant capacity of wheat,increase the content of osmotic adjustment-related substances to maintain high water potential,and increase the hormone content to enhance wheat tolerance to cross stress.Drought priming enhanced wheat stress tolerance by regulating leaf osmotic adjustment under cross stress,while different priming mainly enhances waterlogging tolerance by regulating the overall antioxidant balance of wheat plants and leaf hormones.